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Catalyst 2950 Desktop Switch Software Configuration Guide
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Chapter 26 Configuring QoS Unde r sta ndi n g QoS
•Only one policer can be applied to a packet in the input direction.
•Only the average rate and committed burst parameters are configurable.
•Policing occurs on the ingress interfaces:
–
60 policers are supporte d on ingress Gigabi t-capa ble Ether net ports.
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6 polic er s are suppo rt ed o n i ngre ss 10/10 0 E therne t p ort s.
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Granula rity for the averag e burst r ate i s 1 Mbps for 10 /100 po rts a nd 8 Mbps for Gi gabi t
Ether net ports .
•On an interface configured for QoS, all traffic received through the interface is classified, policed,
and marked according to the policy map attached to the interface. On a trunk interface configured
for Qo S, tr affic in all VLANs received through the interface is classified, policed, and marked
according to the policy map attached to the interface.
Note You ca nno t c onfigure po l icers on th e egre ss inte rfa ces.

Mapping Tables

Note Th is fea ture is available o nly i f your sw itch i s r unn ing the EI .
During classification, QoS uses a configurable CoS-to-DSCP map to derive an internal DSCP value from the
received CoS value. This DSCP value represents the priority of the traffic.
Before the traffic reaches the scheduling stage, QoS uses th e configurable DSCP-to-CoS map to derive
a CoS v alu e fro m the inte rnal DSCP v alue . The CoS v alu e is u sed to se lect on e of the four e gress queues.
The CoS- t o-DSCP an d DS CP-t o-C oS map s h ave default values that mi gh t or mig ht n ot be ap pro pria te
for y our n etwor k.
For configuration information, see the “Configuring CoS Maps ” section on pa ge 26-24.

Queuein g an d Sc he dulin g

Note Both the EI and SI support this feature.
The sw itch provide s QoS- based 80 2. 1P CoS values. QoS use s cl assification a nd sched uling to send
network t raffic from t he sw it ch i n a pr ed icta ble mann er. QoS cla ssifies fra mes by assi gnin g
priority-indexed CoS values to them and gives preference to higher-priority traffic such as telephone
calls.

How Class of Service Works

Before you set up 802.1P CoS on a Catalyst 2950 switch t hat operates with the Catalyst 6000 fa mily of
switches, refer to the Catalyst 6000 documentation. There are dif ferences in the 802.1P implementation
that you sh ould understand to ensure compatibility.